Aromatic dicarboxylic acids are well known starting materials for making polyester resins, which polyester resins are used widely as principal polymers for polyester fibers, polyester films, and resins for bottles and like containers. For a polyester resin to have properties required in many of these uses, the polyester resin must be made from a polymer grade or "purified" aromatic acid, such as purified terephthalic acid.
Purified terephthalic acid is derived from relatively less pure, technical grade or "crude" terephthalic acid by purification of the latter utilizing hydrogen and a noble metal catalysts as described in U.S. Pat. No. 3,584,039 to Meyer. In the purification process, impure terephthalic acid is dissolved in water at an elevated temperature, and the resulting solution is hydrogenated, preferably in the presence of a hydrogenation catalyst, e.g., palladium on a carbon support, as described in U.S. Pat. No. 3,726,915 to Pohlmann. This hydrogenation step converts various color bodies present in the relatively impure terephthalic acid to colorless products. Another related purification-by-hydrogenation process of aromatic polycarboxylic acids produced by liquid phase catalyst oxidation of polyalkyl aromatic hydrocarbons is described in U.S. Pat. No. 4,405,809 to Stech et al.
Depolymerization of polyethylene terephthalate by hydrolysis at a high temperature and pressure in the absence of a base or acid, or a catalyst, is known, see for example, U.S. Pat. No. 4,521,556; U.S. Pat. No. 4,587,502; U.S. Pat. No. 4,605,762; GB. Patent No 2,123,403; U.S. Pat. No. 4,578,510; U.S. Pat. No. 4,620,032; U.S. Pat. No 4,626,598; Japanese Patent No. 49020147; and Japanese Pat. No. 56118420. Depolymerization of polyethylene terephthalate by hydrolysis under conditions of neutral pH can, however, result in production of oligomeric co-products (U.S. Pat. No. 4,578,510); derivatives of terephthalic acid (Wolkma Chem., 13(2), 144-55); and/or cyclic trimers (Japanese Patent No. 56118420). Additionally, depolymerization product of waste polyethylene terephthalate in the form of bottles, film, fiber and other manufactured articles usually contain dyes and contaminants (U.S. Pat. No. 4,521,556; GB. Patent No 2,123,403; and Japanese Patent No. 49020147). Accordingly, although various processes are available for hydrolyzing polyethylene terephthalate waste, the purification of recovered terephthalic acid typically requires several process steps to remove dyes, pigments, and other impurities including inorganic compounds such as catalyst residues and organic compounds which can result from depolymizeration reactions.
U.S. Pat. No. 4,335,175, to Pusztaszeri, exemplifies some difficulties encountered in preparing a purified terephthalic acid from polyethylene terephthalate waste. Polyester scrap such as film (with or without silver), fabric, yarn, or bottles, was depolymerized at ambient temperatures with a mixture of concentrated sulfuric acid and water to form crude terephthalic acid. Pusztaszeri states that an alkaline solution, which can be dark brown or black in color, containing crude terephthalic acid resulting from the depolymerization, is filtered to obtain a clear liquid which many be light brown in color (if colored, it must be treated with activated charcoal and filtered from the charcoal). The resulting solution is then acidified with sulfuric acid to precipitate the terephthalic acid. Terephthalic acid is then recovered by filtration and washed.
In U.S. Pat. No. 5,051,528 to Naujokas and Ryan, a method is described for recovering ethylene glycol and forming dimethyl terephthalate from polyethylene terephthalate waste by dissolving the scrap in a solvent consisting of oligomers of the same monomers at atmospheric pressure and passing super-heated methanol through the solution at temperatures below 270.degree. C. Dimethyl terephthalate and ethylene glycol are said to be carried out of the solution with the flow of super-heated methanol. Methanol is recovered overhead from the product vapor stream in a first distillation column. In a subsequent distillation column, dimethyl terephthalate and ethylene glycol are separated from a bottom effluent of the first distillation column. "However, aromatic carboxylic acids cannot be obtained directly by this method because of the use of super-heated methanol resulting in formation of dimethyl terephthalate.
In a later filed European Patent Application No 484 963 A2, in the name of Everette, a method is described for obtaining ethylene glycol vapor and forming dimethyl terephthalate vapor by treating polyester polymer with excess methanol vapors at a temperature above 230.degree. C. The excess methanol is said to act as a carrier gas for the ethylene glycol vapor and dimethyl terephthalate vapor. At a pressure of 110 psig the reported yields of dimethyl terephthalate were, even with an excess of at least 3 moles of methanol for every mole of dimethyl terephthalate in the vapor, in a range downward from 88 based on PET content of the starting material. At lower pressures, lower yields were reported for this process. Again, aromatic carboxylic acids cannot be obtained directly by this method because of the use of excess methanol resulting in formation of dimethyl terephthalate.
Recently, in U.S. Pat. No. 5,095,145, to Rosen, a process is disclosed for preparing a purified terephthalic acid from waste polyethylene terephthalate. Scrap was depolymerized at temperatures of from about 221.degree. C. to about 316.degree. C. with water at pressures sufficient to maintain a liquid phase and, subsequent to cooling, a crude terephthalic acid filter cake was recovered from the resulting solution and washed. The cake was reslurried and dissolved in water. Thereupon, the solution obtained was catalytically hydrogenated at temperatures of from about 221.degree. C. to about 316.degree. C. at pressures sufficient to maintain a liquid phase for a period of up to 8 hours. Rosen states that pellets of green waste polyethylene terephthalate from waste green bottles were depolymerized by this process at 274.degree. C. and samples of crude terephthalic acid filter cakes taken after 2 hours and a longer period. After filter cakes of terephthalic acid from green bottles were analyzed for color, L*-values of 91.54 and 68.18, a*-values of -0.55 and 1.22, and b*-values of 5.22 and 15.88, respectively, were reported. In Example XII of U.S. Pat. No. 5,095,145 it is stated that hydrogenation of crude terephthalic acid from waste green polyethylene terephthalate required up to about 6 hours to reduce initial b*-values greater than 2 but less than 10 to less than 2. The reported L*-value, however, increased to over 95 and a*-values also increased, but remained negative.
Regardless of the methods of depolymerization and purification of resulting terephthalic acid, the variable nature of crude terephthalic acid obtained from depolymerization of polyethylene terephthalate waste from many sources and the variable nature of impurities resulting therefrom and contained in the crude terephthalic acid, the process control and thus quality assurance of the purified terephthalic acid, has been made difficult and costly. Because of this lack of quality assurance and its cost relative to that of virgin purified terephthalic acid, purified terephthalic acid from polyethylene terephthalate waste has not been considered as a viable economic replacement for fiber grade virgin purified terephthalic acid prepared from para-xylene.
It is therefore a general object of the present invention to provide an improved method which overcomes the aforesaid problem of prior art methods, for recovery of aromatic acid from polyester resin which has been used for polyester fibers, polyester films, and resins in bottles and like containers.
More particularly, it is an object of the present invention to provide an improved method for recovery from polyester resins aromatic acid sufficiently free of undesired impurities so that the acid can be used to make polyester resins which have good color and other properties needed in manufacture of commercial articles.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims.